Heretofore, polyisocyanates have been reacted with a polyol or a polyamine to form respectively a urethane or a urea.
According to the present invention, it is advantageous when a urethane prepolymer contains a significant amount of unreacted polyisocyanate generally added to increase the overall "hard segment" content of the formulation. An additional advantage can be realized by adding an increment of the chain extender to the urethane prepolymer to form a polyisocyanate/extender adduct which is insoluble in the urethane prepolymer. By this technique, the molecular weight of the adducts is held to a minimum as compared to the conventional procedure of adding all of the chain extender to a prepolymer at one time.